Package-on-Package Structure and Methods for Forming the Same

ABSTRACT

A method includes coining solder balls of a bottom package, wherein top surfaces of the solder balls are flattened after the step of coining. The solder balls are molded in a molding material. The top surfaces of the solder balls are through trenches in the molding material.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a divisional of U.S. patent application Ser. No.13/874,821, entitled “Package-on-Package Structure and Methods forForming the Same,” filed May 1, 2013, which application claims thebenefit of U.S. Provisional Application No. 61/776,747, entitled“Package-on-Package Structure and Methods for Forming the Same,” filedMar. 11, 2013, which applications are hereby incorporated herein byreference.

BACKGROUND

In an existing Package-On-Package (PoP) packaging process, a bottompackage is formed first, which comprises a device die bonded to apackage substrate. A molding compound is then molded on the packagesubstrate, wherein the device die is molded in the molding compound. Thepackage substrate further comprises solder balls that are on the sameside of the package substrate as the device die. The solder balls areused for connecting the bottom package to a top package. Through-moldingopenings are then formed in the molding compound, and hence the solderballs are exposed through the through-molding openings. In the formationof the through-molding openings, laser drill is used to remove portionsof the molding compound that cover the connectors. The laser drilling isa low throughput process. Although the solder balls may be made largeenough, so that the molding compound does not fully cover the solderballs, due to the large size of the solder balls, the pitch of thesolder balls is also large. In addition, the stand-off height betweenthe top package and the bottom package is also high.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1 through 8 are cross-sectional views of intermediate stages inthe manufacturing of a Package-on-Package (PoP) structure in accordancewith some exemplary embodiments;

FIGS. 9 through 13 are cross-sectional views of intermediate stages inthe manufacturing of a PoP structure in accordance with alternativeembodiments, wherein a pin-type mold chase is used to coin solder balls;

FIGS. 14 through 18 are cross-sectional views of intermediate stages inthe manufacturing of a PoP structure in accordance with yet alternativeembodiments, wherein a trench-type mold chase is used to coin solderballs; and

FIGS. 19 through 23 are cross-sectional views of intermediate stages inthe manufacturing of a PoP structure in accordance with yet alternativeembodiments, wherein a terrace-type mold chase is used to coin solderballs.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable concepts that can be embodied in a wide varietyof specific contexts. The specific embodiments discussed areillustrative, and do not limit the scope of the disclosure.

A package and the method of forming the same are provided in accordancewith various embodiments. The intermediate stages of forming thepackages are illustrated in accordance with some embodiments. Thevariations of the embodiments are discussed. Throughout the variousviews and illustrative embodiments, like reference numbers are used todesignate like elements.

Referring to FIG. 1, package components 10 are provided. Packagecomponents 10 may be comprised in package component 100, which comprisesa plurality of package components 10. In some embodiments, packagecomponents 10 are package substrates. Hence, throughout the description,package components 10 are referred to as package substrates, althoughthey may be another type of package components. Accordingly, packagecomponent 100 may be package substrate strip. In alternativeembodiments, package components 10 are interposers.

In some embodiments, package substrates 10 comprise substrates 11 thatare formed of a semiconductor material such as silicon. Alternatively,substrates 11 are formed of a dielectric material. Substrates 11 mayalso be laminate substrates which include laminated dielectric films.Package substrates 10 are configured to electrically couple connectors12 on first surface 10A to conductive features 16 on second surface 10B,wherein surfaces 10A and 10B are opposite surfaces of package substrates10. Conductive features 16 may be metal pads, for example. Packagesubstrates 10 may include metal lines/vias 14 therein. Alternatively,features 14 comprise through-vias penetrating through substrates 11.

Package components 20 are bonded to package substrates 10 throughelectrical connectors 12. Package components 20 may be dies, and henceare alternatively referred to as dies 20 hereinafter, although they mayalso be another type of package component such as packages. Dies 20 maybe device dies comprising integrated circuit devices (not shown) such astransistors, capacitors, inductors, resistors, and the like. The bondingof dies 20 to connectors 12 may be a solder bonding or a directmetal-to-metal bonding (such as copper-to-copper bonding). Underfill 18may be dispensed into the gap between dies 20 and package substrates 10in some embodiments. In alternative embodiments, no underfill isdispensed into the gap between dies 20 and package substrates 10.Throughout the description, package substrates 10, dies 20, and solderballs 24 are in combination referred to as bottom packages 102.

Solder balls 24 are formed on the top surface of package substrates 10.Solder balls 24 may be electrically coupled to connectors 12 andconductive features 16. In accordance with some embodiments, solderballs 24 are placed on package substrates 10, followed by a reflow stepto join solder balls 24 with package substrates 10. Accordingly, solderballs 24 have round surfaces. In some embodiments, top ends 24A ofsolder balls 24 are higher than top surfaces 20A of dies 20. Inalternative embodiments, top ends 24A of solder balls 24 aresubstantially level with, or lower than, top surfaces 20A of dies 20.

Referring to FIG. 2, spacer 26 is placed on package substrate strip 100.Although spacer 26 is illustrated as including discrete portions, theillustrated discrete portions may be portions of an integrated spacer.In some embodiments, spacer 26 has a top-view shape of a grid, with dies20 aligned to the grid openings of the grid. Also, solder balls 24 arealso aligned to the grid openings of the grid. The top ends of spacer 26are lower than the top ends of solder balls 24.

As also illustrated in FIG. 2, rigid board 28 is placed over, andaligned to, solder balls 24. In some embodiments, rigid board 28comprises a rigid material such as metal, ceramic, or the like.Furthermore, rigid board 28 may be formed of a thermal conductivematerial. For example, rigid board 28 may be formed of stainless steel.Similar to spacer 26, although rigid board 28 is illustrated asincluding discrete portions, the illustrated discrete portions may beportions of an integrated board. In some embodiments, rigid board 28 hasa top view shape of a grid, with dies 20 aligned to the grid openings inthe grid. Also, rigid board 28 comprises portions overlapping spacer 26.The bottom surface of rigid board 28 is flat (co-planar).

Coin head 30 is placed over rigid board 28. In some embodiments, coinhead 30 has a top-view size similar to the top-view size of rigid board28, and has a flat bottom surface. Coin head 30 is connected to amechanism (not shown) that is configured to drive coin head 30 down topress rigid board 28. In some embodiments, coin head 30 is configured toheat rigid board 28. For example, coin head 30 may include an electricalcoil (not shown) therein.

Referring to FIG. 3, coin head 30 is operated to press rigid board 28down, so that rigid board 28 presses, and flattens the top surfaces ofsolder balls 24. The action of pressing and flattening the top surfacesof solder balls 24 is referred to as “coining” solder balls 24. In someembodiments, during the time period rigid board 28 is pressed, coin head30 heats rigid board 28, which further heats solder balls 24. Theresulting temperature of solder balls 24 is higher than the roomtemperature (about 21° C. to about 25° C., for example), and lower thanthe melting temperature of solder balls 24. In some exemplaryembodiments, the temperature of solder balls 24 is between about 50° C.and about 150° C. during the coining process. In alternativeembodiments, the heating of solder balls 24 is performed by heatingpackage substrates 10 from bottom.

With the heating of solder balls 24, the required force needed to coinsolder balls 24 is reduced. With the pressing-down of coin head 30, theheight of solder balls 24 is reduced, and rigid board 28 is lowered,until rigid board 28 lands on spacer 26, at which time, the coining maybe stopped. Spacer 26 thus acts as the stopper of the coining.Furthermore, the thickness of spacer 26 defines the resulting height ofsolder balls 24. Spacer 26, rigid board 28, and coin head 30 may then beremoved. The resulting bottom packages 102 are illustrated in FIG. 4.

After the coining, as illustrated in FIG. 4, solder balls 24 have thecross-sectional view of barrels, and have substantially flat topsurfaces and substantially flat bottom surfaces. Furthermore, the middlesections of solder balls 24 have diameters greater than the diameters ofthe top surfaces and the bottom surfaces.

Referring to FIG. 5, molding material 32 is molded on dies 20 andpackage substrates 10, and is then cured. In some exemplary embodiments,molding material 32 comprises a polymer, which may be a moldingcompound, an underfill, a Molding Underfill (MUF), or the like. Solderballs 24 are buried in molding material 32.

FIG. 6A illustrates the laser trimming step to remove the portions ofmolding material 32 that covers solder balls 24. The energy of the laserused in the laser trimming process is adjusted, so that molding material32 is trimmed when exposed to the laser, while solder balls 24 are notremoved even exposed to the laser. As shown in FIG. 6A, as a result ofthe laser trimming, trenches 34 are formed, with solder balls 24 exposedto trenches 34. The resulting molding material 32 includes surfaces 32Aoutside of trenches 34, and surfaces 32B inside trenches 34. Surfaces32B are lower than surfaces 32A. In addition, top surfaces 24A of solderballs 24 may be substantially level with or higher than top surfaces 32Bof molding material 32.

Solder balls 24 have maximum diameters D1, which are greater than thediameter D2 of the top surfaces 24A. Since solder balls 24 have thebarrel shapes, in the upper half of each of solder balls 24, thediameters of solder balls increase with the reduction in height. In someembodiments, the laser trimming process is controlled, so that theportions of solder balls 24 over surfaces 32B have diameters smallerthan maximum diameter D1. Alternatively stated, the laser trimming maybe controlled, so that the top surfaces 32B of molding material 32 arenot lowered to the level at which solder balls 24 have the maximumdiameter D1.

The laser trimming may have various types. FIGS. 6B and 6C illustratetwo possible types of trimming. FIGS. 6B and 6C are top views of one ofbottom packages 102. Referring to FIG. 6B, in some embodiments, trenches34 in the same bottom package 102 are interconnected to form largetrenches. Furthermore, all trenches 34 in the same bottom packages 102may be interconnected to form a trench ring, as shown in FIG. 6B. Thetrench ring may encircle die 20. The cross-sectional view of bottompackage 102 is similar to what is shown in FIG. 6A.

FIG. 6C illustrates another type of trimming, in which each of solderballs 24 is exposed through a discrete trench 34, and the discretetrenches 34 are disconnected from each other. In the illustratedexample, trenches 34 have square top view shapes. In alternativeembodiments, trenches 34 may have other types of top view shapesincluding, and not limited to, circular shapes, hexagon shapes, octagonshapes, and the like.

Referring to FIG. 7, top packages 40 are placed onto bottom packages102. Solder balls 42 of top packages 40 are aligned to, and are placedon, solder balls 24. Each of top packages 40 may include device die 44,and package substrate 46, on which device die 44 is bonded. Furthermore,molding material 48 may be molded on device die 44.

Next, as shown in FIG. 8, a reflow is performed, so that top packages 40are bonded to package substrates 10. After the reflow, solder balls 24and 42 (FIG. 7) are molten and joined, and the resulting solder regionsare referred to as solder regions 50 in FIG. 8. In some embodiments,solder regions 50 have a profile in which the lower halves are widerthan the respective upper halves. For example, the maximum diameter D1is in the lower half of each of solder regions 50. In some embodiments,the maximum diameter D1 of solder regions 50 occur at height H1, whichis close to a quarter of the over height H2 of solder regions 50. Afterthe reflow, an underfill (not shown) may be disposed between toppackages 40 and bottom packages 102. A sawing step may be performedalong scribe lines 51. The structure in FIG. 8 is hence sawed into aplurality of packages 200.

FIGS. 9 through 13 illustrate the cross-sectional views of intermediatestages in the formation of a PoP structure in accordance withalternative embodiments. Unless specified otherwise, the materials andformation methods of the components in these embodiments are essentiallythe same as the like components, which are denoted by like referencenumerals in the embodiments shown in FIGS. 1 through 8. The detailsregarding the formation process and the materials of the componentsshown in FIGS. 9 through 13 (and in the embodiments shown in FIGS. 14through 23) may thus be found in the discussion of the embodiments shownin FIGS. 1 through 8.

Referring to FIG. 9, initial packages 102 are provided. The bottompackages 102 are essentially the same as shown in FIG. 1, and hence thedetails are not discussed herein. Next, referring to FIG. 10, mold chase52 is placed on bottom packages 102, and is pressed against solder balls24. Mold chase 52 may be formed of stainless steel, ceramics, copper,aluminum, or other type of rigid materials. In some embodiments, moldchase 52 includes pins 52A that extend down to contact, and to press on,solder balls 24. Pins 52A may be identical to each other. Pins 52A aredesigned to align to solder balls 24, so that each of solder balls 24corresponds to, and may be aligned to, one of overlying pins 52A. Insome embodiments, pins 52A have vertical sidewalls with the top portionsand the bottom portions having the same shape and same widths. Inalternative embodiments, pins 52A have a tapered profile, with the lowerportions of pins 52A have horizontal dimensions (width) smaller than thehorizontal dimensions of the upper portions of pins 52A. The number ofpins 52A may be equal to the total number of solder balls 24. The bottomsurfaces of pins 52A are flat. Mold chase 52 further includes body 52B,to which pins 52A are attached.

As also shown in FIG. 10, force 54 is applied to press mold chase 52down. Accordingly, solder balls 24 are coined, and the top surfaces ofsolder balls 24 are flattened. The resulting solder balls 24 may haveessentially the same profile presented in the embodiments shown in FIG.4. During the coining of solder balls 24, solder balls 24 may be heated,for example, to a temperature between about 50° C. and about 150° C.

Next, referring to FIG. 11, with mold chase 52 remain to be on bottompackages 102, polymer 32 is injected into the space between mold chase52 and bottom packages 102. When injected, polymer 32 is in a liquidform and is able to flow. Molding compound 32 may be in contact with thetop surface and the edges of dies 20, and may be in contact with pins52A, solder balls 24, and the top surfaces of package substrates 10. Thetop surface 32A of molding compound 32 may be higher than top surfaces20A of dies 20, and dies 20 may be fully encapsulated in moldingcompound 32. Molding compound 32 is then cured, for example, in athermal curing process, although other curing methods may be used. Moldchase 52 may then be taken away, and the molding process is finished.

FIG. 12 illustrates bottom packages 102 after mold chase 52 is removed.Trenches 34 are left where pins 52A (FIG. 11) were located, and the topsurfaces of solder balls 24 are exposed. Trenches 34 may have sizes andshapes substantially the same as each other. The top view of trenches 34may be found to be similar to what is shown in FIG. 6C. In a subsequentstep, as shown in FIG. 13, top packages 40 are bonded to bottom packages102 through solder regions 50, which are formed by reflowing solderballs 24 (FIG. 12) and the solder balls in top packages 40. The profileof solder regions 50 may be essentially the same as in FIG. 8, and isnot repeated in detail herein. A sawing step may then be performed alongscribe lines 51, so that the structure shown in FIG. 13 is sawed into aplurality of packages 200. Alternatively, the sawing may be performedafter a subsequent filling of a molding material.

FIGS. 14 through 18 illustrate the cross-sectional views of intermediatestages in the formation of a PoP structure in accordance with yetalternative embodiments. These embodiments are similar to theembodiments in FIGS. 9 through 13, except that instead of having pins52A corresponding to solder balls 42 with a one-to-one correspondence,all solder balls 24 in each of package substrates 10 is coined by thesame pin, which form a ring. Referring to FIG. 14, bottom packages 102are provided. The bottom packages 102 are essentially the same as shownin FIG. 1, and hence the details are not repeated herein.

As shown in FIG. 15, mold chase 52 is pressed on solder balls 24. Moldchase 52 includes body 52B, and a plurality of pin rings 52A, which arephysically disconnected from each other. Each of the pin rings 52Aoverlaps all solder balls 24 of one of bottom packages 102. The bottomsurfaces of all of pin rings 52A are co-planar. Mold chase 52 is presseddown, so that solder balls 24 are coined to have barrel shapes.

Next, referring to FIG. 16, molding material 32 is injected into thespace between mold chase 52 and bottom packages 102, and is cured. Moldchase 52 is then taken away, as shown in FIG. 17, the resulting trenches34 forms a plurality of trench rings, each in one of bottom packages102. Furthermore, each of trenches 34 encircles the respective die 20that is in the same bottom packages 102. The top view of theseembodiments may be essentially the same as in FIG. 6B. FIG. 18illustrates the bonding of top packages 40 to bottom packages 102.

FIGS. 19 through 23 illustrate the cross-sectional views of intermediatestages in the formation of a PoP structure in accordance with yetalternative embodiments. These embodiments are similar to theembodiments in FIGS. 14 through 18, except that pin rings 52A (FIG. 20)overlying each of bottom packages 102 join each other to form a pingrid.

Referring to FIG. 19, an initial package is provided. The initialpackage is essentially the same as shown in FIG. 1, and hence thedetails are not discussed herein. Next, as shown in FIG. 20, mold chase52 is placed to coin solder balls 24. Mold chase 52 includes body 52B,and a mold grid 52A underlying and attached to body 52B. Mold grid 52Amay be considered as the pin rings 52A in FIG. 15 merge with each other.The grid lines of mold grid 52A overlap all solder balls 24 of allbottom packages 102. The grid openings of mold grid 52A are aligned todies 20. A grid line of mold grid 52A may include a first halfoverlapping a first one of bottom packages 102, and a second halfoverlapping a second half of a second one of bottom packages 102. Asalso shown in FIG. 20, mold chase 52 is pressed down, so that solderballs 24 are coined. Next, referring to FIG. 21, molding material 32 isinjected into the space between mold chase 52 and bottom packages 102,and is cured. Mold chase 52 is then taken away, as shown in FIG. 22.

FIG. 23 illustrates the bonding of top packages 40 to bottom packages102. A sawing step may then be performed along scribe lines 51 to sawthe structure in FIG. 23 into a plurality of packages 200.Alternatively, the sawing may be performed after a subsequent filling ofa molding material. Packages 200 in FIG. 23 are similar to the packagesin FIG. 18, except that in FIG. 23, each of packages 200 includes aportion of molding material 32 overlapping die 20, with this portion ofmolding material 32 having top surface 32A, All remaining top surfaces32B of molding material 32 are lower than top surface 32A. Furthermore,the top surface 32B, which is the top surface of molding material 32 intrench 34, extends all the way to the edge of package 200. In FIG. 18,however, top surface 32B in each of packages 200 forms a ring, and doesnot extend to the edge of the respective package 200. Alternativelystated, in FIG. 18, trench 34 (and top surface 32B) is between a firsttop surface 32A (denoted as 32A1) that is directly over die 20, and asecond top surface 32A (denoted as 32A2) that extends to the edge ofpackage 200.

In the embodiments of the present disclosure, by coining solder balls,the top surface of solder balls have increased areas. Accordingly, theprocess window for aligning the solder balls in the top packages to thebottom packages is enlarged. With solder balls being coined, the heightsof the solder balls in the final PoP packages are reduced.

In accordance with some embodiments, a method includes coining solderballs of a bottom package, wherein top surfaces of the solder balls areflattened after the step of coining. The solder balls are molded in amolding material. The top surfaces of the solder balls are throughtrenches in the molding material.

In accordance with other embodiments, a method includes placing a spaceron a package substrate of a bottom package, and coining a plurality ofsolder balls of the package substrate using a co-planar surface thatcontacts the plurality of solder balls. Top surfaces of the plurality ofsolder balls are flattened by the co-planar surface. The step of coiningis stopped after the co-planar surface is stopped by the spacer. Themethod further includes removing the spacer, molding the plurality ofsolder balls in a molding material, and removing a top portion of themolding material over the plurality of solder balls to reveal theplurality of solder balls.

In accordance with yet other embodiments, a package includes a bottompackage, which includes a package substrate, and a die over and bondedto the package substrate. A plurality of solder regions is disposed overand bonded to a top surface of the package substrate. A molding materialis over the package substrate and molding the solder regions. Themolding material has a first top surface overlapping the die, and atrench extending from the first top surface into the molding material,wherein the plurality of solder regions is exposed to the trench.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method comprising: pressing a mold chaseagainst solder regions of a bottom package to coin the solder regions,wherein top surfaces of the solder regions are flattened by thepressing; encapsulating the solder regions that have flattened topsurfaces in an encapsulating material, wherein the encapsulatingmaterial is dispensed into a space between the bottom package and themold chase; and removing the mold chase.
 2. The method of claim 1further comprising, during the pressing, heating the solder regions to atemperature lower than a melting temperature of the solder regions. 3.The method of claim 1 further comprising curing the encapsulatingmaterial when the mold chase is in contact with the encapsulatingmaterial.
 4. The method of claim 1, wherein the mold chase comprises aplurality of discrete pins, and in the pressing, each of the pluralityof discrete pins is pressed against one of the solder regions.
 5. Themethod of claim 1, wherein the mold chase comprises a plurality of pinrings separate from each other, and wherein during the pressing, allsolders regions of the bottom package are pressed by one of theplurality of pin rings.
 6. The method of claim 1, wherein the mold chasecomprises a pin grid, and wherein during the pressing, all soldersregions of a plurality of bottom packages are pressed by the pin grid.7. The method of claim 1 further comprising bonding a device die ontothe bottom package, wherein after the pressing, top surfaces of thesolder regions are lower than a top surface of the device die.
 8. Themethod of claim 1 further comprising, after the removing the mold chase,bonding a top package to the bottom package, with the top package joinedwith the solder regions.
 9. A method comprising: bonding a device dieonto a bottom package, wherein the bottom package comprises solderregions on a same side of the bottom package as the device die; pressingpins of the mold chase against the solder regions; dispensing anencapsulating material into a gap between the mold chase and the bottompackage; taking away the mold chase to expose the solder regions; andbonding a top package to the solder regions.
 10. The method of claim 9,wherein the pressing the mold chase results round top surfaces of thesolder regions to be flattened as planar top surfaces.
 11. The method ofclaim 10, wherein the planar top surfaces are lower than a top surfaceof the device die.
 12. The method of claim 10, wherein during thepressing, the solder regions are heated.
 13. The method of claim 9,wherein in the pressing, the pins are pressed against the solder regionswith a one-to-one correspondence.
 14. The method of claim 9, wherein thepins are in form of a plurality of pin rings separate from each other,and wherein during the pressing, all solders regions of the bottompackage are pressed by one of the plurality of pin rings.
 15. The methodof claim 9, wherein the pins are portions of a pin grid, and whereinduring the pressing, all solders regions of a plurality of bottompackages are pressed by the pin grid.
 16. A method comprising: bonding adevice die onto a bottom package, wherein the bottom package comprises aplurality of solder regions; encapsulating the device die and the bottompackage with a encapsulating material, wherein the encapsulatingmaterial comprises: a first planar top surface directly overlying thedevice die; and a plurality of second planar top surfaces, wherein eachof the second planar top surfaces is coplanar with a planar top surfaceof one of the plurality of solder regions; and bonding a top package tothe bottom package through the plurality of solder regions.
 17. Themethod of claim 16 further comprising, before the encapsulating, coiningthe plurality of the plurality of solder regions.
 18. The method ofclaim 17, wherein the coining is performed by pressing the plurality ofsolder regions using a mold chase.
 19. The method of claim 18, whereinthe mold chase comprises a plurality of flat bottom surfaces, with eachof the flat bottom surfaces pressed against one of the plurality ofsolder regions.
 20. The method of claim 18 further comprising dispensingthe encapsulating material into a gap between the mold chase and thebottom package.